Apolipoprotein B is the principal apolipoprotein on chylomicrons, very low density lipoproteins, intermediate lipoproteins, and low density lipoproteins. Apolipoprotein B exists in human plasma in two forms, the full length apoB-100 and the shorter apoB-48. ApoB-100 is synthesized primarily by the liver, whereas apoB-48 is derived exclusively from the small intestine. Both forms of the protein are requisite for the normal transport and delivery of dietary and endogenous lipids to target tissues. Both proteins are the product of a single gene which encodes a mRNA that may either be directly translated into the full-length apoB-100 or, alternatively, be modified by conversion of single nucleotide that results in the replacement of the codon CAA at amino acid 2153 with the premature STOP codon UAA. This modification results in the shorter translation product apoB-48 and is referred to as mRNA "editing." By analysis of mRNA derived from human and rat tissues at various stages of development, our laboratory this year has demonstrated that the capacity to edit apoB mRNA is developmentally controlled in a tissue and species-specific manner. Furthermore, we have demonstrated that one of the requisite developmental tissue-specific signals is thyroid hormone: in the absence of thyroid hormone normal editing does not develop in the liver, while the small intestine edits normally; administration of thyroid hormone can restore normal editing in the developing liver.